The Randomized Intervention for children with Vesicoureteral Reflux (RIVUR) Study was initiated in 2005 to assess the efficacy of antimicrobial prophylaxis in children between birth and 6 years of age diagnosed with vesicoureteral reflux (VUR) following a febrile urinary tract infection (UTI). Six hundred children identified following one or two febrile or symptomatic UTIs, who are identified as having VUR using voiding cystourethrography (VCU), are randomized to antimicrobial prophylaxis with trimethoprim/sulfamethoxazole (TMP/SMZ) or placebo. Children that are entered into the study are evaluated with initial dimercaptosuccinate (DMSA) renal scan to determine for the presence of renal scarring. Follow-up evaluation is performed with telephone contacts every 2 months and clinic visits every 6 months. VCU is performed at study end to determine persistence/resolution of VUR. DMSA renal scan is performed at 12 months and at study end at 24 months to determine new onset of renal scarring. Additionally, development of antibiotic resistance is assessed with the use of fecal cultures obtained at follow-up visits. Primary study aims are to determine if the use of antibiotic prophylaxis (i) reduces the incidence of UTI or delays the time to recurrent UTIs in children with VUR, (ii) increases the development of resistant E. coli or increases the incidence of UTIs caused by E. coli and (iii) leads to reduction in the development of renal scarring. The protocol of the RIVUR study was developed as a consensus between the investigators from five core clinical centers and the data coordinating center along with input from the NIH/NIDDK. As of October 2009, 370 children have been recruited into the study and are being followed for 2 years. The primary aim of the extension is to complete recruitment and follow-up of the study cohort.